contd...
The particulate emission reduction by DOC is influenced by the exhaust gas temperature as shown on Fig. 6. 11. The optimum temperature range for the DOC operation is observed to be from about 200 to 350º C. At lower temperatures poor oxidation of SOF and PAH is obtained and at temperatures higher than 350º C a high conversion of SO2 to sulphates results in an increase of mass of PM emissions.
| Figure 6.11 |
Effect of exhaust gas temperature on conversion of particulate mass by DOC. |
The diesel fuels during early 1990s contained 0.2 to 0.3 % sulphur by mass Due to high fuel sulphur content the DOC design has to address to the following requirements;
- Minimize conversion of SO2 to SO3 at high exhaust gas temperatures
- Minimize formation and storage of the sulphate on the catalyst.
- Good conversion of SOF so that DOC reduces the mass of PM emissions in addition to conversion of HC and CO.
As sulphur in the diesel fuels has been reduced to around 0.03% the sulphate formation on DOC is not of serious concern. .
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